1. Field of the Invention
The present invention relates to inverters and other DC loads operating from DC power sources such as batteries. In particular, the present invention relates to such systems which include filter capacitor banks at the direct current inputs of the DC loads.
2. Description of the Prior Art
Conventional inverters, and other DC loads, often use large capacitor banks at their direct current (DO) inputs to filter out internal switching transients and noise. Capacitor banks are used to prevent these unwanted signals from appearing on the DC bus or in the batteries or other DC power source.
However, when such loads are connected to the DC bus, an excessive inrush of current can occur as the capacitors charge. This capacitor charging current may greatly exceed the normal operating current for the system and cause damage to the capacitors, cabling and/or connection devices if not limited. Current limiting devices, however, should not be allowed to interfere with the operating demands of the DC load.
Conventional DC loads with large input capacitor banks are therefore often precharged, when first connected to the DC power source, by means of auxiliary relays or contacts which connect the capacitor banks through current limiting devices. Thereafter, when the capacitor banks have been fully or almost fully charged, the DC bus is directly to the DC load, bypassing the current limiting devices.
Similarly, when the DC load is disconnected, auxiliary discharging devices are connected to safely remove the charge from the capacitor banks. In systems using high voltage DC power sources, such charges can present lethal voltages capable of persisting for relatively long durations, if not properly discharged.
Such conventional precharging and discharging systems, however, include auxiliary relays, contactors and other expensive components and tend to consume power and therefore reduce system performance. Mechanical portions of such components tend to have short operating lives and can hang up and refuse to connect or disconnect. These problems substantially reduce system reliability and present important safety problems for the operation of such systems.
A fail-safe technique is required which permits mating of the main power contacts only after the capacitor bank charging has been safely accomplished and which automatically and reliably discharges the capacitor bank when and only when the main and auxiliary connections have been broken.